I started with Blissey but have since moved to Eviolite Chansey as a waller. That said, here's a fun Serene Grace Blissey I made that can really wreck people's day with a little luck:
Blissey @ King's Rock / Leftovers
Ability: Serene Grace
EVs: 106 HP / 252 Def / 152 SpD
Bold Nature
- Charge Beam / Calm Mind
- Fire Blast
- Ice Beam / Sweet Kiss
- Soft-Boiled
This Blissey retains her defensive nature while being able to do effective special damage to most Pokemon types, with each offensive move also coming with a good chance of a secondary effect (Flinch, Burn, Frozen, guaranteed Spec. Attack boost). Burn is excellent as it lower's the opponent's attack stat, making Blissey much harder to kill.
EDIT: After using this Blissey in a good number of battles now I have to say it's not bad at all, in fact I've been using it over Chansey pretty often as a rating battle Special Defense choice. If you can get just two Charge Beams off, she really becomes a great attacker.